Help optimizing code?

[Lily]

I started learning D a few days ago, coming from some very basic 
C++ knowledge, and I'd like some help getting a program to run 
faster. The code is here: 
https://github.com/IndigoLily/D-mandelbrot/blob/master/mandelbrot.d

Right now it runs slower than my JavaScript Mandelbrot renderer 
on the same quality settings, which is clearly ridiculous, but I 
don't know what to do to fix it. Sorry for the lack of comments, 
but I can never tell what will and won't be obvious to other 
people.